This invention generally relates to apparatus and methods for plugging tubular members and more particularly relates to a tube plug for plugging a heat exchanger tube, such as a nuclear steam generator tube, which tube plug has a reduced susceptibility to stress corrosion cracking.
Although devices and methods for plugging nuclear steam generator tubes are known in the art, it has been observed that these devices and methods have a number of operational problems associated with them. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a nuclear steam generator.
A nuclear heat exchanger or steam generator generates steam when heat is transferred through heat conductors from a heated, pressurized and radioactive primary fluid to a non-radioactive secondary fluid. The primary fluid flows through a plurality of tubes (i.e., a tube bundle) as the secondary fluid flows across the exterior surfaces of the tubes which are disposed in the steam generator. The walls of the tubes function as the heat conductors for conducting heat from the primary fluid flowing through the tubes to the secondary fluid flowing across the exterior surfaces of the tubes.
Because the primary fluid flowing in the steam generator tubes is radioactive, the steam generator is designed such that the radioactive primary fluid flowing in the tubes does not radioactively contaminate the nonradioactive secondary fluid flowing across the tubes. Therefore, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid.
Occasionally, due to tube wall intergranular cracking caused by stress and corrosion during operation (i.e., known in the art as primary water stress corrosion cracking), the steam generator tubes may degrade (i.e., experience tube wall thinning) and develop surface and volume flaws and thus may not remain leak-tight. If through-wall cracking occurs due to the flaws or degradation, the radioactive primary fluid may commingle with the nonradioactive secondary fluid. Thus, the steam generator tubes are typically non-destructively inspected to determine if any tubes have flaws. If flaws are indicated, the suspect tube is removed from service such as by plugging the open ends of the degraded tube. On the other hand, the tube, although degraded, may remain in service by sleeving the degraded portion of the tube. In either case, the radioactive primary fluid is prevented from commingling with the non-radioactive secondary fluid even though the wall of the tube is degraded.
As stated hereinabove, a degraded tube may be plugged to remove it from service. A tube plug for this purpose comprises an "INCONEL" tubular shell that is open on one end and closed at the other end, and which contains a frustoconically shaped expander member. The expander element is a conical wedge and is disposed completely within the interior of the shell with its larger circular end facing the inner surface of the closed distal end of the plug shell. Instead of being perfectly cylindrical, the interior walls of the shell are slightly tapered by increasing the thickness of the shell walls from the distal closed end to the proximal open end. When the wedge is forcefully pulled from the closed end toward the open end of the shell, it will radially expand the plug into sealing engagement with the inner wall of the steam generator tube by a wedging action. Such a plug is disclosed by U.S. Pat. No. 4,390,042 issued Jun. 28, 1983 in the name of Harvey D. Kucherer et al. entitled "Tube Plug" and assigned to the Westinghouse Electric Corporation. Although this tube plug will satisfactorily plug the tube, the inner surface of the plug shell is nonetheless exposed to the pressurized primary fluid which will pass through the open end of the plug shell and contact the inner surface of the plug shell. As explained hereinbelow, this is undesirable because the primary fluid may corrosively attack the inner surface of the plug shell under certain conditions and cause cracks to occur in the shell. Such a cracked shell may then allow the radio-active primary fluid to commingle with the nonradioactive secondary fluid.
In this regard, it has been observed that when a steam generator tube is plugged with a tube plug composed of "INCONEL" material which has undergone a particular annealing process, the tube plug itself may experience primary water stress corrosion cracking because the inner surface of the tube plug is exposed to the primary water. If through-wall cracks develop in the tube plug, the tube plug may not remain leak-tight; therefore, the cracked tube plug may allow the radioactive primary fluid to commingle with the nonradioactive secondary fluid. Moreover, if the tube plug cracking is severe, portions of the tube plug may fracture away from or separate from the tube plug and be propelled along the inside of the steam generator tube due to the pressure of the primary water acting against the interior surface of the tube plug. It has been observed that propulsion of the separated portion of the tube plug in the tube may severely damage the tube in which the tube plug is disposed. Therefore, it is desirable to provide a tube plug that has a reduced susceptibility to stress corrosion cracking.
Hence, although devices and methods for plugging tubular members are known in the art, the prior art does not appear to disclose a tube plug for plugging a tubular member wherein the tube plug has a reduced susceptibility to stress corrosion cracking.
Therefore, what is needed is a tube plug for plugging a tubular member wherein the tube plug has a reduced susceptibility to stress corrosion cracking.